Method for impressing gratings within fiber optics

ABSTRACT

An optical fiber has a dielectric periodic index of refraction phase grating established in its core by intense angled application of several transverse beams of ultraviolet light, enabling the establishment of a distributed, spatially resolving optical fiber strain gauge.

This is a division of application Ser. No. 925,512 filed on Oct. 27,1986 which was a continuation application of U.S. Ser. No. 640,489,filed Aug. 13, 1984, now abandoned.

TECHNICAL FIELD

This invention relates to impressing, establishing, printing or writingphase gratings in optical fibers or waveguides and the optical detectionand measurement of strain distributions with multi-wavelength lightprovided to said phase gratings.

BACKGROUND OF THE INVENTION

It is known to determine the distribution of axial strain or temperaturealong the length of a fiber optic sensor according to the techniquedescribed by S. K. Yao et al. in 21 Applied Optics (1982) pages3059-3060. According to this technique, very small deformations at theinterface between an optical core and its cladding will cause lightmeasurably to couple from core to cladding modes. This permitsmeasurements by time-domain reflectometry or a series of cladding tapsto determine transmission loss and the distribution of appliedperturbations.

DISCLOSURE OF INVENTION

According to the invention, phase gratings are impressed along the coreof an optical waveguide by the application of intense beams ofultraviolet light transverse to the axis of the core at selected anglesof incidence and the complements thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of the spatially resolving optical fiberstrain gauge according to the invention addressed herein;

FIGS. 2A through 2C are partial schematics of selected sections of theoptical waveguide including its cores, indicating grating patterns ofvarying spacing corresponding to selected regions A, B and C in amechanical structure being monitored for strain;

FIG. 3 is a graph of the intensity spectrum of the reflected lightproduced by injecting broadband light into the core of the waveguidewith shifts in the spectral lines indicating strain at specificstations; and

FIG. 4 shows a schematic illustration of a technique for establishing agrating pattern of variable spacing at selected positions along thelength of the optical waveguide.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a schematic diagram of the spatially resolving opticalfiber strain gauge 13. The gauge 13 includes an optical waveguide 15 orfiber operative to transmit a single or lowest order mode of injectedlight.

The core 19 of waveguide 15 is preferably a germanium-doped silica orglass filament. The core 15 contains a series of variable spacing Braggreflection gratings 16 written, impressed or otherwise applied byapplication of a variable two-beam ultraviolet (less than 300 nanometer)interference pattern. These periodic gratings 16 or refractive indexperturbations are permanently induced by exposure to intense radiation.

FIGS. 2A through 2C shows the establishment of different wavelengthgratings 16 corresponding to respective locations on core 19.

Each of selected gratings 16 is formed by transverse irradiation with aparticular wavelength of light in the ultraviolet absorption band of thecore material associated with a position in a structural component 22.This procedure establishes a first order absorption process by whichgratings 16 each characterized by a specific spacing and wavelength canbe formed by illuminating core 19 from the side with two coplanar,coherent beams incident at selected and complementary angles theretowith respect to the axis of core 19. The grating period is selected byvarying the selected angles of incidence. Thus, a permanent change inthe refractive index is induced in a predetermined region of core 19, ineffect creating a phase grating effective for affecting light in core 19at selected wavelengths.

As indicated in FIG. 1 the optical waveguide 15 and core 19 are attachedor embedded in a section of structural component 22, in particular aplate for example. Core 19 contains characteristic periodic refractiveindex perturbations or gratings 16 in regions A, B and C thereof. Abroadband light source 33 or tunable laser is focused through lens 33'onto the exposed end of core 19. A beam splitter 34 serves to direct thereturn beam from core 19 toward a suitable readout or spectrometer 37for analysis. Alternatively, a transmitted beam passing out of the end19' of core 19 could be analyzed.

The spectrum of the reflected light intensities from strain gauge 13 isshown in FIG. 3. A complementary transmitted spectrum is alsoestablished passing out of the end 19' of core 19. The spectrum containsthree narrowband output lines centered at respective wavelengths:lambda_(A), lambda_(B) and lambda_(C). These output signals arise byBragg reflection or diffraction from the phase gratings 16 at respectiveregions A, B and C. In this example, regions A and C of structuralcomponent 22 have been strained by deformation, causing a compressionand/or dilation of the periodic perturbations in the fiber core 19.

As a result, the corresponding spectral lines are shifted as shown inFIG. 3 to the dotted lines indicated. The respective wavelengthdifferences delta lambda_(A) and delta lambda_(C) are proportional tostrain in respective regions A and C.

FIG. 4 illustrates the formation of periodic perturbations or gratings16 in a region of fiber core 19 in response to exposure of core 19 tointense transverse ultraviolet radiation. Grating spacings Δa and Δc arecontrolled by the incidence angle of incident interfering beams 99 andbeam 101. As can be seen, the angles of incidence of beams 99 arecomplements (i.e. their sum equals 180 degrees) to each other withrespect to the axis of core 19. The incident pair of beams 99 can bederived from a single incident beam 101 passing in part through a beamsplitter 103 and reflecting from spaced parallel reflectors 105. Byincreasing the separation between reflectors 105 and correspondinglyvarying the angles of incidence of beam 101, the angles of incidence ofbeams 99 upon core 19 can be controlled. Accordingly, the fringe spacingin grating 16 is varied as desired along the length of core 19, topermit a determination of strain or temperature corresponding tolocation along gauge 13.

Several spacings can be superimposed or colocated by this technique forthe response set forth below.

Sensitivity to external perturbations upon structural component 22 andthus also upon core 19 depends upon the Bragg condition for reflectedwavelength. In particular, the fractional change in wavelength due tomechanical strain or temperature change is: ##EQU1## q is thethermooptic coefficient, which is wavelength dependent; αis theexpansion coefficient;

εis the axial or longitudinal strain;

lambda_(i) is the wavelength reflected by the grating at location ialong the core 19;

n is the refractive index of the optical waveguide; and

ΔT is the change in temperature.

This relationship suggests a way to compensate for temperature changesalong the length of the fiber sensor. In particular, if superimposedgratings of different spacings are provided, each of the two gratingswill be subject to the same level of strain, but the fractional changein wavelength of each grating will be different because q is wavelengthdependent.

Accordingly, each pair of superimposed gratings will display acorresponding pair of peaks of reflected or transmitted intensity.Accordingly, the shifts of these peaks due to a combination oftemperature and strain can be subtracted. The shifts in these peaks dueto strain will be the same in magnitude. Accordingly, any remainingshift after subtraction is temperature related. Thus, when it is desiredto know the strain difference as between several locations possiblysubject to a temperature difference, the temperature factor can becompensated.

The relationship therefore permits compensation for temperaturevariation during measurement, since the photoelastic and thermopticeffects are wavelength dependent. In other words, by superimposing twoor more gratings at each location of interest, two or more spectrallines are established at each point of measurement. Strain will affectboth lines equally; temperature will not. Thus, sufficient informationis available to permit determination of the magnitude of strain and thetemperature difference.

The information above is likely to cause others skilled in the art toconceive of other variations in carrying out the invention addressedherein, which nonetheless are within the scope of the invention.Accordingly, reference to the claims which follow is urged, as thosespecify with particularly the metes and bounds of the invention.

We claim:
 1. An optical fiber having a single core including at leastone grating with a predetermined grating spacing permanently provided inat least one grating region of the core by exposing the core to aninterference pattern resulting from mutual interference of two beams ofultraviolet radiation simultaneously directed at the fiber at suchdifferent acute angles of incidence relative to the longitudinal axis ofthe core that the interference pattern has fringes situated at saidpredetermined grating spacing from each other and propagatestransversely through the core with attendant permanent change in theindex of refraction of the core in correspondence with the interferencepattern.
 2. The optical fiber according to claim 1, wherein saiddifferent acute angles of incidence complement one another to 180° withrespect to the longitudinal axis of the core.
 3. The optical fiberaccording to claim 2, wherein the core includes at least one additionalgrating similar to and provided in the same manner as said one gratingbut having a predetermined grating spacing different from that of saidone grating.
 4. The optical fiber according to claim 3, wherein saidadditional grating is provided at said grating region.
 5. The opticalfiber according to claim 3, wherein said additional grating is providedin an additional grating region that is longitudinally spaced from saidone grating region.